Thermionic cathodes are well known in the electronics industry. Presently, there are two basic types of thermionic cathodes: (1) the impregnate-type, and (2) the top-layered type. The difference between the two types of thermionic cathodes is their physical structure.
Generally, impregnant-type thermionic cathodes are composed of a porous billet impregnated with a semiconductor having a predetermined composition of materials that react with the billet material, when heated, to generate electron emissions. Layered-type cathodes, on the other hand, are generally composed of a base material having a surface upon which a layer of emissive material is formed. The base can be an impregnated billet or even a multi-layered material as long as the base and the top layer react, when heated, to emit electrons.
Specific examples of cathodes formed by this general model are described in several U.S. Patents issued to some of the inventors herein. The following is a list of some of these U.S. Patents, all of which are incorporated herein by reference: U.S. Pat. No. 5,114,742, entitled, "Method of Preparing an Improved Scandate Cathode;" U.S. Pat. No. 5,074,818, entitled, "Improved Scandate Cathode;" U.S. Pat. No. 4,895,699, entitled, "Barium Peroxide, Iridium and Excess Tungsten as Impregnants for Cathodes;" U.S. Pat. No. 4,840,767, entitled, "Method of Making a Cathode from Tungsten and Iridium Powders Using a Barium Iridiate Formed from Barium Perioxide and Iridium Oxide as the Impregnant;" U.S. Pat. No. 4,818,480, entitled, "Method of Making Oxyanion using BaO.sub.2 with Ir and Os or Rh for Cathode Impregnation;" and U.S. Pat. No. 5,298,830, entitled, "Method Of Preparing An Impregnated Cathode With An Enhanced Thermionic Emission From A Porous Billet And Cathode So Prepared."
Other Patents that illustrate the different compositions: of thermionic cathodes, which are also incorporated herein by reference, include: U.S. Pat. No. 5,118,984, entitled "Electron Tube Cathode," issued Jun. 2, 1992, to Saito et al.; U.S. Pat. No. 4,924,137, entitled "Cathode For Electron Tube," issued May 8, 1990 to Watanabe et al; and U.S. Pat. No. 4,783,613, entitled "Impregnated Cathode," issued Nov. 8, 1988, to Yamamoto et al.
These patents clearly show that the electron emission efficiency of thermionic cathodes directly depends on the chemical mechanism under which the cathode operates. In addition, the Patents show that the chemical mechanism under which the cathode operates depends on the structure and composition of the materials that make up the cathode
For example, the Saito '984 patent teaches of and describes a layered-type cathode composed of three discrete layers of material; a base, a metal layer, and an emissive layer. The materials in these layers react, when heated, to generate electron emissions. More specifically, when the Saito cathode is heated a reducing agent migrates from the base to the emissive layer to initiate electron emissions therefrom. Thus, the Saito cathode depends on a Knudson-type flow mechanism, wherein a reducing agent migrates from a material furthest from the emissive surface of the cathode, to generate electrons.
In contrast, the Branovich '830 patent teaches of and describes a thermionic cathode having a completely different structure, and thus a completely different mechanism form generating electrons. Generally, the Branovich cathode utilizes an emissive material composed of at least one oxygen deficient compound and at least one fully oxidized material which react, when heated, to enhance the electron emissions of the cathode. More specifically, the Branovich cathode is structured such that this emissive material is located as close to the emissive surface of the cathode as possible so that when the cathode is heated, the oxygen deficient material reacts with the fully oxidized material to generate electron emission at lower temperatures than, for example, the Saito cathode. Moreover, as electrons are generated the emissive material goes through a chemical reaction which constantly regenerates the oxygen deficient material, and thus provides improved electron emissions over the prior art.
In copending application, U.S. patent application Ser. No. 08/218,533, entitled "Improved Thermionic Cathode and Method of Making The Same," filed Mar. 28, 1994, the applicants of the present invention disclosed that the emission of the cathode, utilizing the Branovich mechanism as described above, depends upon the formation of oxygen deficient compounds and their ratio to the oxygen sufficient or fully oxidized materials. Moreover, it has been shown in all cases that the amount of oxygen sufficient or fully oxidized material must be greater than the amount of oxygen deficient material, and that the optimum ratio is not the same for all combinations of oxygen deficient and fully oxidized material.
In addition, in the above cited co-pending application, the inventors of the present invention disclosed several ways emissions could be maximized, including: (1) Adding an oxygen deficient compound to an impregnant; (2) Adding compounds, such as Aluminum Tungstate, Al.sub.2 (WO.sub.4).sub.3, or Scandium Tungstate, Sc.sub.2 (WO.sub.4).sub.3, which in the presence of Tungsten, W, react to form oxygen deficient compounds, such as WO.sub.2 and AlWO.sub.4 or ScWO.sub.4 ; and (3) Adding composites of 1 and 2 above.
Thus, the applicants have disclosed that adding Aluminum Tungstate Or Scandium Tungstate could enhance emissions through the Branovich mechanism. However, it has not been disclosed that a mechanism for enhancing emissions exists for a cathode having a mixture of Aluminum Tungstate and Scandium Tungstate.